It is known that whatever their kind and origin, fats and oils have limited stability. During storage they undergo various deteriorative reactions that reduce their nutritive value and also produce volatile compounds, giving off unpleasant smells and tastes. In general, the term rancidity has been used to describe the mechanisms by which lipids alter in nature, mechanisms that may have a biological or chemical origin. Alterations of a biological nature include those produced by microorganisms (e.g., bacteria, fungi and yeasts) that may be inhibited by the addition of preservatives, and those produced by enzymes, mainly hydrolytic rancidity or lipolysis. The latter may be inhibited by thermal treatment, by conservation at low temperature, or by reducing the percentage of water.
Alterations of a chemical nature are due to the action of oxygen. Lipid oxidation reactions, known as autooxidation, commonly occur in lipids with a high content of unsaturated fatty acids and constitute the most common deterioration of fats and oils. However, unsaturated fatty acids are not the only constituents in foods that undergo oxidation. Compounds that impart color and taste to foods, like some vitamins, are also susceptible to oxidation.
It has been shown that the oxidation of unsaturated fatty acids takes place through a chain reaction that essentially consists of an initiation or induction stage, which implies the formation of fat free radicals; a propagation stage in which fat free radicals remove a hydrogen atom from a lipid to form a relatively stable hydroperoxide and a new unstable fat free radical. These hydroperoxides may interact with proteins, pigments, and other food constituents to generate substances whose chemical nature may be harmful to human health. As a final step in autooxidation, the hydroperoxides split into smaller short chain organic compounds such as aldehydes, ketones, alcohols, and acids which cause the off-odors and off-flavors characteristic of rancid fats and oils.
In plants, the most widespread polyunsaturated fatty acids are linoleic acid (Omega-6) and alpha-linolenic acid (Omega-3). Many vegetable oils contain Omega-6 fatty acid (linoleic acid). However, unlike many other vegetable oils, flaxseed oil also contains significant amounts (generally about 55 to about 65 percent) of Omega-3 fatty acid (alpha-linolenic acid). Their presence in food is of great importance since they cannot be synthesized by human and animal tissues and should thereby be provided with the diet. In tissues these essential fatty acids are converted to longer and more unsaturated fatty acids of the Omega-6 and Omega-3 families, such as arachidonic acid (AA), eicosapentaenoic (EPA), and docosahexaenoic (DHA), which are present in fish oil in relatively high amounts. The health benefits of linoleic acid, alpha-linolenic acid, AA, EPA and DHA are well documented in the literature. These benefits include hypolipidemic, anti-thrombotic, and anti-inflammatory properties. They are also essential fats for growth, brain function, and visual acuity, especially for infants. The degree of unsaturation of highly unsaturated fatty acids makes them extremely sensitive to oxidation, resulting in lipid peroxide and subsequent development of off flavors, odors, and dark color, which decrease the nutritive value of polyunsaturated oils and related food. The rate at which the oxidation reaction proceeds depends on several factors such as temperature, degree of unsaturation of the lipids, oxygen level, ultraviolet light exposure, presence of trace amounts of pro-oxidant metals (i.e., iron, copper, nickel), lipoxidase enzymes, and so forth. Flaxseed oil and fish oil can become rancid in few weeks or less, even if refrigerated.
The presence of certain chemical compounds may inhibit the process of lipid oxidation. The term “antioxidants” in foods is usually applied to those compounds that interrupt the chain reaction involved in autooxidation. Primary antioxidants are those mainly phenolic antioxidants, that interrupt the chain of free radicals and among which are found natural and synthetic antioxidants such as tocopherols, butylated hydroxyanisol (BHA), butylhydroxytoluene (BHT), tertiary butylated hydroquinone (TBQH), and propyl gallate. All of them act as donors of electrons.
It has been long recognized that various acids, and some of their derivatives, provide apparent antioxidant effect when added to vegetable oils. These are commonly referred to as acid-type antioxidants. However, these acids, if added alone to oils containing no primary antioxidant, will exhibit virtually no effect on the oxidative stabilities of the oil. It is believed that the acids are not truly antioxidants but more likely function by enhancing, in some manner, the activity of primary antioxidants naturally present (such as tocopherol) in the oils, or those synthetic antioxidants that are added. Common acid-type antioxidants include ascorbic acid, ascorbyl palmitate, and erithorbic acid. Unlike the primary antioxidants that function as electron donors, ascorbic acid and ascorbyl palmitate function by the entirely different mechanism of oxygen scavenging. Pro-oxidation occurs in lipid-based systems containing certain metal ions and reducing agents. Casein has been shown to act as a non-reducing agent by oxidizing iron from its ferrous to the ferric form (Emery T. in Biochem. Biophys. Res. Comm. 182, 1047–1052 (1992)). Polyphenol-rich extracts from a variety of plant sources e.g., tea, coffee, cocoa, wine, aloe vera, and oak leaves and bark are known to extend the shelf life of products by inhibiting oxidative rancidity.
Polyphenols inhibit free radical formation and the propagation of free radical reactions through the chelation of transition-metal ions, particularly those of iron and copper (Brown et al. in Biochem. J 330, 1173–1178 (1998)). Citric acid, amino acids, and ethylenediaminetetraacetic acid also form chelates with metallic ions such as copper and iron, thus avoiding their catalytic action on the oxidation of lipids. Most of these chelating agents exhibit little or no antioxidant activity when used alone, and therefore they are considered as synergistic agents of other antioxidants. Thus, they increase, to a great extent, the action of primary antioxidants.
Numerous extracts from plants and spices such as rosemary, sage, thyme, oregano, cloves, ginger, mace and nutmeg, exhibit antioxidant activity. However, these natural antioxidants are not very effective and suffer from the disadvantage of having intensive characteristic herb and spice flavors, which may limit their use in some applications. Many different natural antioxidant compositions have been developed over the years. Natural antioxidant compositions are typically blends of ascorbic acid (vitamin C), tocopherol (vitamin E), citric acid, rosemary extract, and phospholipids (i.e., soybean lecithin, egg yolk lecithin). Ascorbyl palmitate is also used in these natural antioxidant compositions. For example, U.S. Pat. No. 5,077,069 discloses a complex of tocopherol, ascorbic acid, citric acid and phospholipids that are useful in preventing oxidation of oils. U.S. Pat. No. 5,102,659 discloses a complex of ascorbyl palmitate, a mixed tocopherol concentrate, and rosemary extract useful for prolonging the shelf life of vitamin/dietary supplements which are highly susceptible to rancidity. U.S. Pat. No. 5,230,916 discloses an ascorbic acid complex for stabilizing polyunsaturated oil. U.S. Pat. No. 5,258,179 discloses the use of coenzyme Q in combination with ascorbic acid and phospholipid to provide protection from oxidation. U.S. Pat. No. 5,427,814 describes the use of a mixture of tocopherol, lecithin, and ascorbic acid to protect lipids against oxidation.
These natural antioxidant compositions also suffer from problems that limit their usefulness. Thus, the combination of ascorbic acid and lecithin (an ionic phospholipid) is known to produce an undesirable red color in the oil. A high amount of lecithin may also impart an undesirable odor and flavor to the product. Ascorbic acid is ineffective as an antioxidant in hydrophobic substrates. Esters of ascorbic acid with saturated fatty acids particularly ascorbyl palmitate and ascorbyl stearate are used instead. However, these fat-soluble ester derivatives are exceptionally costly and do not fall within the narrow definition of natural. It is also costly to remove objectionable solvents used to dissolve oil-insoluble compounds present in these natural antioxidant compositions.
Many products susceptible to oxidation are emulsions or may be made into emulsions. An emulsion is a colloidal dispersion of two immiscible liquids, such as oil and water, in the form of droplets. If oil droplets are finely dispersed in water, then this is an oil-in-water or “O/W” emulsion. When water droplets are finely dispersed in oil, then this is a water-in-oil or “W/O” emulsion. O/W and W/O emulsions play a prominent role in the preparation of a wide range of products including foods, pharmaceutical products and cosmetics. It would be thus desirable to provide antioxidant compositions formed from natural ingredients and methods to effectively reduce oxidation reactions within highly polyunsaturated oils in O/W and W/O emulsions.